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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

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7. Determination of the Illuminating Power.—This requires a
photometer, one of those most largely used being that of Lummer and Brod-
hun (Fig. 45), which is an improved form of the Bunsen type. It consists

FIG. 45.

of a closed chamber with two opposite circular apertures, by means of
which the two faces of a white screen in the chamber are illuminated respec-
tively by the lamp to be examined and by a lamp chosen as unit. The two
faces of the screen reflect the light, by means of a system of lenses, on to
two concentric zones of the field of the telescope
eye-piece shown to the left of the figure. When
the screen is equally illuminated on both faces, the
two zones of the field appear exactly similar. The
two sources of light are placed at the extremities of a
double guide 3 metres in length and graduated in
half-centimetres—the photometric bench. The position
of the photometer is adjusted between the lights so
that the field is uniformly illuminated: the inten-
sities of the two lights are then proportional to the
squares of their respective distances from the screen
of the photometer.

Photometric observations are made in a dark room with blackened
walls, with a temperature about constant and no sensible air currents. The
standard lamp used is the Hefner amyl acetate lamp with a flame 4 cm.
high (Fig. 46).

The values obtained with other types of lamp still in use in different countries
may be converted into those given by the Hefner lamp by means of the factors
given in the following table 1:

FIG. 46.

1 Uppenborn and Monasch : Lehrbuch der Photometrie, Munich and Berlin, 1912. results should not vary by more than 0-5-1°